ALEXASTDER VO^" HTJMBOLBT. 169 menon of the zodiacal light, is described in one part of Humboldt^s diary, written on the voyage from Lima to the western coasts of Mexico. He says: " For three or four nights, between ten and fourteen degrees northern latitude, I saw the zodiacal light in such brilliancy as I have never observed before. Judging by the bril- liancy of the stars and nebulae, the transparency of the atmosphere in this part of the South Sea is very great. From 14th to the 19th of March, no trace of the zodiacal light was visible three quarters of an hour after the sun's disk had set in the sea, although it was completely dark. An hour after sunset, it suddenly became visible in great splendour, between Aldebaran* and the Pleiades ; narrow, long drawn clouds were dispersed over the lovely blue near the horizon, like before a yellow carpet; the upper ones from time to time played into bright colours, as if it were a second sunset. In this region of the firma- ment, the brightness is increased like at the first quarter of the moon. About ten o'clock, the zodiacal light was generally already very faint, and at midnight . only a slight trace was visible/' In our dull, so called temperate, northern zone, the zodiacal light is plainly visible only in the commencement of spring, after twilight, over the western horizon, and toward the end of autumn, before the morning twilight, over the eastern horizon. "We now come to Humboldt's explanation of this extraordinary natural phenomenon, which did not excite the attention of natural philosophers and astro- nomers, until the middle of the seventeenth century., and whose views on the subject are as variously differ- ent as imperfect. Humboldt refutes the hitherto accepted opinion, that the luminous solar atmosphere itself causes the zodiacal light, but considers it pro- bable that its material cause may be the existence o£ a very much flattened ring, formed of vaporous matter, which revolves in space between the courses of Tenus * The "beaufcifal red star in the sign of Taurus, th£ bull.